That's not really true.
Far-right terror was taken very seriously in 1995 after the Oklahoma City bombing. The FBI hired 500 agents specifically to focus on domestic terrorism [0], particularly the then-new militia movement. Virtually every state and federal law enforcement agency established a domestic terror unit. Through continuous effort they'd pretty much quashed the militia movement by 2004, despite the shift of federal resources to the Global War on Terror post-9/11. However, far-right militias surged again in the Obama years [2].
(Much further back, U.S. Grant used the Army to crack down on the Klan [3]. Unfortunately, his successor Rutherford B. Hayes effectively ended Reconstruction and rolled back much of this progress.)
[0] https://web.archive.org/web/20120210125747/http://news.bbc.c... [1] https://web.archive.org/web/20110623153230/http://www.rickro... [2] https://www.npr.org/sections/thetwo-way/2012/03/08/148217754... [3] https://www.historynet.com/grant-takes-klan.htm